A fluid trainer increases the resistance as you pedal faster, the same way that wind resistance increases on the road. A magnetic trainer's resistance doesn't change as much (or at all?) as your speeds increase, so these often have multiple settings, either on the unit or with a cabled remote.

You can increase the workout on a fluid trainer all the way to 10-second sprint levels of effort, just by shifting to a higher gear and increasing your cadence. Or do long steady effort workouts at a lower speed.

The Kurt has a standard power curve, so you can find your watts for a given speed. They say it's the equivalent of a typical sized rider going up a 1% grade. Your bike computer needs to be linked to the back wheel. (It doesn't tell you much about peak watts efforts, but it's still interesting to see how much effort 200 watts, 400, etc watts takes). The other fluid trainers probably work this way, too.